Quantum computers could gain an advantage over their conventional counterparts by bypassing the usual rules of cause and effect.
“We were curious about how to push the limits of quantum computers, making them even more powerful,” says at The Hong Kong University of Science and Technology. To that end, he and his colleagues turned to an odd procedure called a “quantum switch”, which complicates the conventional idea of causality.
According to the rules of cause and effect, if event A caused event B, then event B cannot have caused event A. But in 2009, researchers theorised that applying the quantum switch operation to a system could make it so that “A causes B” and “B causes A” were both true at once, a situation called indefinite causal order. A different group of researchers has since observed the phenomenon in action: in a 2017 experiment, they implemented a quantum switch by making a particle of light go through a series of mirrors and lenses until it became impossible to tell what order of events caused the light to reach its final state.
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In the case of quantum computers, events A and B can be steps in a calculation – as if a computer tried to perform an operation like subtraction or division, but it couldn’t tell which number came first in the equation. Jing and his colleagues formulated a mathematical model for this situation and used it to calculate how a property of qubits known as “magic” changes if a quantum switch is applied to these building blocks of quantum computers.
Qubit states that have a high level of magic “are like super fuel for quantum computers and could help quantum computers solve problems that are otherwise too complex for traditional computers”, says Jing. So boosting the magic of a quantum computer’s qubits and operations is thought to make it more powerful – and the researchers found that the quantum switch provides exactly this kind of boost.
For instance, some programs that can be run on a quantum computer can be easily simulated on a classical machine – which suggests the quantum device wouldn’t be necessary at all. But the researchers’ calculations showed these operations had their magic enhanced when combined with the quantum switch, enabling the quantum computer to calculate in a way that a classical computer could not.
at the University of Calgary in Canada says researchers had previously found that the quantum switch could be a useful ingredient for devices that make very precise measurements of quantum properties or aim to extract work from some quantum process, like tiny quantum engines. “This is yet another example of a situation in which having access to the quantum switch allows us to do things that otherwise we could not perform,” he says.
However, Scandolo says that implementing the ideas from the new study in an actual quantum computer may be difficult. This is because it may not be possible to build a quantum switch that would affect all the computer’s operations at once, which could limit its usefulness.
But Jing is optimistic and believes that some of his team’s results can soon be tested in experiments. He plans to apply it to qubits made from particles of light, which have been used to test a quantum switch before. And there is still more theoretical work to be done. “We may try to establish a whole and better theory on ordering, time [and] information flow in the quantum world,” he says.
Physical Review A
